Power amplifiers are used in a variety of systems including wireless communication systems such as Long Term Evolution (LTE) systems. For example, a base station such as an evolved node B (eNB) has power amplifiers to amplify and transmit a signal on an RF carrier to one or more mobile devices in a region served by the base station. Conversely, a mobile device has a power amplifier to amplify and transmit a signal on an RF carrier to the base station.
Such power amplifiers are typically operated in a non-linear region to achieve high efficiency. Feed forward (FF) power amplifier linearization is a technique that reduces the output distortion of a non-linear power amplifier (PA). A typical FF power amplifier linearization system is shown in FIG. 1. The power amplifier system 10 has two FF cancellation loops: the signal cancellation loop (SCL) 12 and the distortion cancellation loop (DCL) 14. Details of the SCL 12 are not shown in the figure. The DCL 14 includes at least 1 main path and 1 FF path with independent delays 16 and 18. In one of the paths is an adjustable complex gain 20. Successful operation of the SCL 12 prevents the components of the DCL 14 from being pushed into non-linear operation, and prevents the DCL 14 from cancelling the desired signal to be amplified. Successful operation of the DCL 14 linearizes the PA 10 output signal. FIG. 2 shows a typical power amplifier system 11, having an additional FF path in the cancellation loop 14. The additional path includes a delay 22 and a complex gain 24.
The primary performance metrics of the FF cancellation loops 12 and 14 are attenuation level and attenuation bandwidth. The attenuation level is very sensitive to phase and amplitude between the FF paths in the DCL 14. The relationship between phase mismatch and amplitude mismatch is shown in FIG. 3. Adaptive algorithms may be needed to maintain adequate attenuation during environmental changes and component drift. FIG. 4 shows that the attenuation bandwidth is limited by group delay mismatch between the FF paths and by frequency dependent components.
The attenuation bandwidth of the conventional FF amplifier linearizer, DCL 14, can be increased by adding more FF paths in the cancellation look. Each path generally has a different delay element. The outputs of the paths are summed by summers 26. Complex gain adjusters 20 and 24 are used within the FF loops to adjust the magnitude and phase of the signal. In a radio frequency, RF, power amplifier, the signal to be adjusted is at a radio frequency.
The complex gain components 20 and 24 can be realized with a variable gain component, such as a variable gain amplifier, VGA, or voltage variable attenuator, WA, cascaded with a variable phase shifter. Alternatively, the functionality of a complex gain element can be realized with a vector modulator, also known as a vector attenuator. The first function block in a vector modulator splits the input signal into two paths with the two paths differing in phase ideally by 90 degrees. The paths are independently adjusted in magnitude and are then recombined. These approaches to complex gain implementation require frequency independent phase shifters. However, a constant phase shift over a wide frequency band is difficult to realize. Another implementation of the vector modulator involves two paths differing in delay in order to realize the 90 degree phase shift function at some specific frequency. However, this implementation is inherently narrow band.